The Shrink Ray is an unlockable weapon in Big Willy Unleashed.
The Shrink Ray shrinks things like cars, crates, boxes, containers, and humans. You can't shrink buildings, boss level characters, yourself, or the game floor. Also you can't PK anything that got shrunk. Once a human or enemy get shrunk they will not attack you, the humans will run away in terror and vehicles will just stop what it's doing. It's only obtainable after completing all of the story missions and selecting it from the Unlockable Menu.